(a) Field of the Invention
The present invention relates to a zoom lens system, more particularly to a zoom lens system capable of high magnification with a high F-number as well as a high resolution.
(b) Description of the Related Art
Generally, a digital camera uses either a CCD (Charge Coupled Device) or a charge tube of a small charge area to form an image of an object. The digital camera converts the formed image into electrical signals. For the above reason, the digital camera uses bright lenses as picture-taking lenses.
Moreover, the digital camera needs optical members such as an optical low-pass filter, or an infra-red cut filter between the picture-taking lenses and the CCD. Also, the digital camera needs optical devices such as a tricolor separation prism to form each image of the three primary colors of red, green and blue onto each CCD for a three-plate digital camera. Therefore, the digital camera requires a longer back focal length than a focal length to accommodate optical devices, such as the above-mentioned tricolor separation prism.
Also, as the need for photographing moving objects increases, digital cameras have used a high magnification zoom lens system as a picture-taking lens. A general zoom lens system of a digital camera includes a first lens group of a positive refractive power which is fixed, a second lens group of a negative refractive power which is movable along an optical axis for magnification changes, a third lens group of a negative refractive power which is movable along an optical axis to maintain an image plane during magnification changes, and a fourth lens group of a positive refractive power which is fixed.
In recent years, as digital cameras have used electric motors to move the lenses for focusing, it has become necessary to limit the size and weight of the first lens group. Thus, an inner focusing method is generally used. The inner focusing method allows compact and lighter-weight lens groups while maintaining a total length of the optical system, which enables a small motor to move the lens groups.
Conventional four-lens-group zoom lens systems employing inner focusing method are disclosed in Japanese Laid-open Patent Application No. Sho 63-44614, Japanese Laid-open Patent Application No. Sho 61-178917, Japanese Laid-open Patent Application No. Sho 63-123009, Japanese Laid-open Patent Application No. Hei 3-259209, Japanese Laid-open Patent Application No. Hei 5-72474, Japanese Laid-open Patent Application No. Hei 7-270684, and Japanese Laid-open Patent Application No. Hei 8-5920.
Japanese Laid-open Patent Application No. Sho 63-44614, Japanese Laid-open Patent Application No. Hei 3-259209, and Japanese Laid-open Patent Application No. Hei 8-5920 disclose a system of four lens groups that has a first lens group of a positive refractive power viewed from the object side which is fixed, second and third lens groups of a negative refractive power which are movable to change a magnification, and a fourth lens group of a positive refractive power which is fixed.
However, the systems disclosed in Japanese Patent Application No. Sho 63-44614, and Japanese Patent Application No. Hei 3-259209 cannot obtain a high resolution because of a large number of aberrations, and cannot achieve a sufficiently long back focal length.
The system disclosed in Japanese Laid-open Patent Application No. Hei 8-5920 is better than the systems disclosed in Japanese Laid-open Patent Application No. Sho 63-44614 and Japanese Laid-open Patent Application No. Hei 3-259209 in terms of back focal length and optical performance. However, the number of lenses in the moving lens groups requires a more powerful motor to move them when changing the magnification. Moreover, in a telephoto mode, the short distance between the second and the third lens groups, which are the magnification changing group and compensating group, respectively, requires a complex mechanical structure that is not easy to implement.
Japanese Laid-open Patent Application No. Sho 63-123009, Japanese Laid-open Patent Application No. Sho 61-178917, Japanese Laid-open Patent Application No. Hei 7-270684, and Japanese Laid-open Patent Application No. Hei 5-72474 disclose four-lens-group systems comprising a first lens group which is fixed, a second lens group which is movable for changing the magnification, a third lens group which is fixed, and a fourth lens group which is movable for both compensating and focusing.
However, the systems dislcosed in Japanese Laid-open Patent Application No. Sho 63-123009, Japanese Laid-open Patent Application No. Sho 61-178917, and Japanese Laid-open Patent Application No. Hei 7-270684 cannot achieve a high resolution due to a large number of aberrations. Also, a number of lenses in the second and fourth groups of the system disclosed in Japanese Laid-open Patent Application No. Hei 7-270684 makes it difficult for a small motor to move the lenses.
Even though the system disclosed in Japanese Laid-open Patent Application No. Hei 5-72474 shows a good optical performance with a simple structure, its aperture ratio is as small as F-2.0 and its employment of a pair of double sided aspherical lenses in the lens groups makes it difficult to manufacture.